Generally, a semiconductor laser diode is used as a light source for an optical pickup for reproducing optical recording media such as CDs and DVDs.
For satisfactorily reproducing the recording media, semiconductor laser diodes having different light emitting wavelengths and objective lenses with different numerical apertures (NA) should be provided for reproducing CD and DVD, respectively. For example, a laser diode emitting a laser beam at wavelength of 650 nm and an objective lens having NA of 0.6 is used for reproducing DVDs, while a laser diode emitting a laser beam at wavelength of 780 nm and an objective lens having NA of 0.45 is used for reproducing CDs.
For enabling a single player to reproduce different types of discs such as CD and DVD, an optical pickup incorporating a light source for emitting two laser beams having different wavelengths, i.e., 650 nm and 780 nm has been considered. FIG. 1 illustrates an example of such an optical pickup.
The optical pickup illustrated in FIG. 1 comprises a laser diode 1 for emitting a laser beam having a wavelength of 650 nm; a laser diode 2 for emitting a laser beam having a wavelength of 780 nm; a combining prism 3; a half mirror 4; a collimator lens 5; and an objective lens 6. These components are arranged in the above order. Further, on another optical axis branching from the half mirror 4, a photodetector 7 is arranged. This optical system is configured so that an optical path from the laser diode to the combining prism 3 for CD reproduction is different from that for DVD reproduction and an optical path from the combining prism 3 to a recording medium 8 for CD reproduction is substantially identical with that for DVD reproduction. Thus, the light beams emitted from the respective laser diodes travel to the recording medium 8 along an optical axis Y after passing through the combining prism 3.
The above-mentioned configuration, however, requires a large number of elements including the combining prism, which results in a high cost. In addition, the two laser diodes must be aligned with the combining prism, creating a complicated configuration as well as difficulties in adjusting the alignment.
In view of the problem mentioned above, an object of the present invention is to provide an optical pickup apparatus having a simplified configuration for using a plurality of laser beams having different wavelengths to reduce the size of the apparatus.